Armature: Add poll message explaining bone groups need pose mode

Add a poll message to the bone group operators, to explain they only
work in pose mode. Before, the buttons would be greyed out with no
explanation.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D15119

source/blender/editors/armature/pose_group.c

@@ -42,6 +42,7 @@
 static bool pose_group_poll(bContext *C)
 {
   if (!ED_operator_posemode_context(C)) {
+    CTX_wm_operator_poll_msg_set(C, "Bone groups can only be edited in pose mode");
     return false;
   }